HE2.2.22

A Study on the Lateral Structure of Muons
in Extensive Air Showers
with the GRAPES Array at Ooty
Y.Hayashi1, Y.Aikawa1, N.V.Gopalakrishnan2, S.K.Gupta2, N.Ito1, S.Kawakami1, D.K.Mohanty2,K.C.Ravindran2, K.Sasaki1, M.Sasano1, K.Sivaprasad2, B.V.Sreekantan2, H.Tanaka1, S.C.Tonwar2,and K.Viswanathan2
1
Faculty of Science, Osaka City University, Osaka 558 8585, JAPAN2
Tata Institute of Fundamental Research, Mumbai 400 005, INDIA

Abstract
The GRAPES III Experiment has started data taking from the beginning of 1998 with observations on theelectron component with 100 out of the 217 detectors placed in 8m hexagonal configuration. The very large
area muon detector covering 560m2 area is spread over 16 modules (Emu>1GeV) located close together.
Extensive air showers in the size range 104 to 106 have been analyzed to study the lateral distribution ofmuons. Results obtained from this analysis show good agreement with results obtained from earlier
observations at Mt. Norikura. Comparison with results reported by other groups are presented.

1 Introduction:

West - East [m]Observations of muon components in EAS have been done for a long time to study some feature of ultrahigh energy interactions with nuclei and to determine the chemical composition of primary cosmic ray flux.
Most of the muon detectors used in earlier times look as point detector compared with the scale of EAS due
to the limitations imposed by high cost and difficulties in proper maintenance over long periods. It isessential to scale up the muon detectors in area to understand the physical mechanism underlying the knee
region at energy about 3*1015
eV. In addition , to study theshower phenomena in smallsize shower, it is preferable touse densely packed detectors.
Also, in larger size showers, itis important to determine thelateral distribution of muons in
individual showers. The verylarge area muon detector in the
GRAPES III array at Ooty has
been installed to achieve theseobjectives. To determine the
lateral distribution of muons atlarge distances from the core of
EAS, classified by size, it is

essential to observe large
North - South [m]number of muons with goodaccuracy. Muon detectors used
Figure 1: Air Shower Array at Ooty. Small squre showsin the present experiment areelecteon detectors, large squre showers muon detectors.
tracking detectors and satisfiedthe above requirements.

2 Electron and Muon Detectors:

Number of muons by calculation

The GRAPES III array has been designed with a separation of 8m between the adjacent detectors. The
hexagonal shaped array has given good triggering efficiency over the entire range of interest in energy of
3*1013 to 3 *1016 eV. Figure 1 shows the layout of the inner 217 detectors placed around the central
laboratory, with muon detectors located towards the northern edge of the array. Each of these electrondetectors is a 1m2 area, 5cm thick plastic scintillation detector viewed by a fast 5cm diameterphotomultipliers placed 65cm above the scintillator. All the electron detectors are instrumented formeasurement of particle density for determination of shower size, as well as relative arrival time fordetermination of the arrival angle, for individual showers. These detectors are monitored in rate at 100msec
interval. Trigger rate is 8Hz with selection of any 9 detectors out of 60. The accuracy in the determinationof the arrival direction of showers is estimated as 1.5degrees for shower of the size of 105. It has been
known for a long time that a simultaneous study on the electron and muon components of EAS has goodpotential of yielding significant results on the composition for primary cosmic rays. We have installed large
area muon detectors at Ooty in southern India. The detector element is the 6m long proportional counter
with a cross sectional area 10*10 cm2. 58 counters are placed side by side on a concrete platform andcovered with 15cm thick concrete slabs. Four layers of counters are arranged in crossed configuration toobtain quite satisfactory tracking of muons. In total 16 modules of muon detectors covering area of 560 m2
have been fully operational since April 1998. Each module, nearly 36 m2 in area, detects muons ofenergy>1.0 GeV (550 g/cm2, about 20 r.l.). To maintain the detectors in stable, individual channel pulse
width,
any
3folds(3900/sec/module) and 4 folds
143(3300/sec module) rates are1 4
4
1beingalso
monitored
9Totalevent
577
12continuously.
mismatch 64
2 5 151[theta < 60]919
2
10
3 Analysis of data:4
Detailed analysis has been40
1
5
carried out for showers which846
1satisfy the following criterion:
555
3(a) More than 15 channels firing
5660
5out of 80 channels.
1
174(b) Out of the largest 4 signals,
1
less than 1 channel to be on
4
2 180
outer most ring.
161(c) Sum of the particles in the 42131channels should be > 10
12
1
particles.
(d) Out of the largest 6 channels,
0 1
except the farthest, remaining
02468
1012
14
channels should be exist within
Number of Muons by eye fitting
30m in diameter.
With these criteria, the observed
Figure 2: Comparison between eye fitting
shower frequency is 0.24Hz andand automatic calculation for determination
accuracy in the determination ofof number of Muons[/module]
the core position is less than 3m.

1

4.5-4.6
4.6-4.7
4.7-4.82

Muon Density [/m ]On the other hand, muon data
were analyzed that satisfied thefollowing:(a) Core position should hit
within the array except outerring area.
(b) Zenith angle theta<25 degrees.
The number of muons weredetermined by the automaticcounting program. Figure 2 shows
the correspondence between thenumber of muons determined byeye fitting and the countingprogram. Within a miscount of
one muon, the correspondence isalmost 99%. With this program,up to 15 muons/module can bedetermined accurately. For larger
number of muons, we candetermine the number using theionization information recorded
for individual muon counters. Forthe discussion of primary
composition of the shower size Ne< 106, it is sufficient for theformer analysis.

4.8-4.9
4.9-5.0

0.1

0.01

0.001
02060

80

100

Distance from shower core [m]Figure 3: Muon lateral distribution for each shower sizegroups. (5.0 > logNe > 4.5)
1Ooty

5.0 < logNe < 5.1

4 Experimental Results:
2]

Norikura
GreisenMuon Density [/mThere are several possible
ways to study for the primarycosmic ray composition throughmuon components, for example,
the multiplicity distribution, theNe-Nmu relation etc. Simulation
results show that in Ne-Nmu
relation, the ratio of muon numbervs shower size gives a ratio of
about 3 between in case of ironand proton primary. In thismethod, it is important to observe
accurately the lateral distributionof muons for showers, which aregrouped according to shower size.
Figure 3 shows the lateral
distribution of muons obtained
from the Ooty experiment forshower size of 5.0 > logNe > 4.5

40

Akeno

0.1

0.01

0.001
020

40

60

80

Distance from shore core [m]Figure 4: Muon lateral distribution compare with otherexperimental results.

100(at 20m < r < 100m). Even the number of showers is 1.6*106 , we can obtain quite high statistics on muon
due to the compact but huge array of muon detectors.

5 Discussion and Conclusions:
To compare the present results with earlier observations by other groups, we show in Figure 4 the lateral
distribution of muons for the size range of 5.1 > logNe > 5.0, from Mt. Norikura experiment, and the Akeno
experiment. Also shown is the parametrisation given by Greisen. The table below also shows thecomparison between experimental data from different experiments.

GreisenAkeno
Norikura
KASCADE
Ootya.s.l.(m)
0900
2770
110
2200

Size Range
5-77-95-7
4.6 - 5.6
4.5 - 5.5

Table
Range of distance(m)10 - 100
103 - 3*103
20 - 50
20 - 100
20 - 100

Detectors
Proportional Counter
Neon Flash Tube
Streamer Tube
Proportional CounterEth(GeV)
110.45
2
1

From the Figure and Table, we can conclude that the Ooty result agree with Mt. Norikura andKASCADE data but Akeno data gives slightly higher in density. This one may come the detectorconfiguration and too much extrapolation are given here in the shower size.
Finally, these observations show that the large area muon detectors in the GRAPES III array permit
accurate determination of muon numbers with high statistics. These observations are expected to give us
reliable information on primary composition of cosmic ray flux. Also, these observations would help indistinguishing showers due to gamma ray in search carried out for discrete sources of ultra-high energy
gamma rays.

Acknowledgement
We are thankful to the Ministry of Education, Japan for partial financial support for this experiment. We arealso happy to acknowledge valuable contributions of S.T. Arasu, A.P. Amalaraj, G. Paul Francis,V.Jeyakumar, K. Manjunath, K. Ramadass, B.S. Rao, C. Ravindran, P. Sumathi, V. Viswanathan andT.Matsuyama during the installation, operation and maintenance of the instrumentation. The help andcooperation of the Radio Astronomy Centre for providing site facilities for the GRAPES III array are
gratefully acknowledged.

References
Greisen K., 1960, Ann.Rev.Nucl.Part.Sci. 10, 63
Sakuyama H., 1972, J. Phys. Soc. Japan 32, 3
Hayashida N., 1995, J. Phys G21, 1101
Gils H.J., 1994 LBL-35822, 418-421
Weber J.H., 1997 25th ICRC HE 1-3, 153